1. Field of the Invention
The present invention relates to a signal processing apparatus and a computer-readable recording medium for recording a signal processing program that determines a color conversion processing for approximating a color signal from an image pickup device to be processed to a color signal of a target image pickup device.
2. Description of the Related Art
Many image pickup apparatuses such as digital cameras and video cameras currently on the market are provided with an image pickup device with a primary color or complementary color filter disposed on a front thereof. However, spectral characteristics are known to differ depending on a type of color filter or a type of image pickup device. Such a difference in spectral characteristics causes color reproduction of an outputted image to vary depending on the image pickup device.
Furthermore, there are individual differences in spectral characteristics even in color filters and image pickup devices of a same type, and therefore color reproduction of outputted images may slightly differ even among image pickup devices of the same type using color filters of the same type.
A color management system (hereinafter referred to as “CMS”) is a technique for absorbing such differences in color reproduction and aiming at realizing identical color reproduction. The CMS generally performs color conversion processing such as matrix conversion and table conversion so that a color signal to be processed matches a target color signal.
When color conversion processing through matrix conversion in the above-mentioned is performed, if a linear relationship holds between a color signal to be processed and a target color signal, high accuracy color reproduction can be realized at low cost, but if no linear relationship holds, high accuracy color reproduction cannot be expected.
On the other hand, when color conversion processing through the above described table conversion is performed, high accuracy color reproduction can be realized irrespective of whether or not a linear relationship holds between the color signal to be processed and the target color signal, but a large capacity memory is required to store table data, leading to increases in cost and power consumption of apparatuses.
To realize high accuracy color conversion processing at low cost while avoiding the above described problems, it is necessary to perform color conversion processing which is based on and suitable for such a relationship between the color signal to be processed and the target color signal. Examples of techniques for solving the problems include Japanese Patent Application Laid-Open Publication No. 2001-358960, which describes a technique of deriving a conversion coefficient for realizing linear color conversion from a signal characterized by a first spectral sensitivity distribution to a signal characterized by a second spectral sensitivity distribution based on the first spectral sensitivity distribution and the second spectral sensitivity distribution. This technique calculates a conversion coefficient based on the first spectral sensitivity distribution and the second spectral sensitivity distribution and thereby provides an advantage of being able to realize stable color conversion processing independent of an illuminating light source or an object.